1. Field of the Invention:
This invention relates to new and useful improvements for making hollow articles such as containers, closures, trays, and the like, from thermoplastic synthetic resinous film or sheeting and, more particularly, to a method for using an expandable plug or plunger which cooperates with a forming mold in forming apparatus to provide hollow articles which during forming can be programed or controlled, as desired, with respect to thickness distribution throughout the walls thereof.
2. Description of the Prior Art:
At the present time, a variety of hollow articles are formed from thermoplastic synthetic resinous materials. They include containers, closures for containers, trays and the like which are generally formed by injection molding or mechanical drawing processes.
In a mechanical drawing process, plastic sheeting or film may be excessively stretched thereby resulting in the formation of hollow articles having an objectionable variable wall thickness, particularly in deep corner areas, or an objectionable appearance due to visible flow lines in the walls thereof. These problems are caused by the sheeting or film sticking to the forming mold thereby producing thick wall sections where the mold first contacts the sheeting or film and excessively thin wall sections in deep drawn corner areas. In extreme cases, hollow articles will have deep drawn areas with very thin wall thicknesses which reduce their marketability because of inadequate strength, impact resistance, ruggedness and/or unpleasant appearance. Furthermore, relatively thick sheeting or film may be necessary to overcome these problems, thereby producing hollow articles which cannot economically compete with similar articles formed from other structural materials such as metal.
To overcome the above problems, a plug or plunger is often used to assist in the formation of mechanically drawn hollow articles. For example, a plug or plunger is often used to push heat softened plastic sheeting or film into a female forming mold of a vacuum forming machine. While the general utilization of plugs or plungers in cooperation with forming molds may improve the formation of hollow articles, the known plug or plunger designs have not been entirely satisfactory in eradicating the material distribution problems in the walls of deep drawn hollow articles and, in some instances, have created other problems such as plug mark off and contamination of the formed articles. In this regard, the previously discussed sticking of the sheeting or film to a mold may also occur on a plug or plunger causing the formation of hollow articles with excessively thick bottom walls and thinned out deep corner areas.
Of the known types of plugs or plungers, the most commonly used is a solid metal or wooden plug which is somewhat smaller than and approximately the same shape as the forming cavity in the mold. This type of plug is illustrated in U.S. Pat. No. 2,891,280. Other known plugs which attempt to improve material distribution in the walls of hollow articles by deforming during processing are disclosed in U.S. Pat. Nos. 2,966,873 and Re 24,833. Plugs with felt or fabric covers and plugs which are lubricated have also been used to improve slip of heat softened plastic sheeting or film over the plug thereby providing better wall thickness distribution in the resulting hollow articles formed therewith.